1. Field of the Invention
The present invention relates generally to a line pressure control arrangement for automatic automotive transmissions and more specifically to a line pressure control arrangement which enables the appropriate control of line pressure during a plurality of different shifting operations.
2. Description of the Prior Art
An automatic transmission Service Manual (publication A261C07) issued in March 1987 relating to the so called RE4RO1A type transmission describes an arrangement which utilizes tabled data for controlling the line pressure during shifting and non-shifting modes of transmission operation. In this arrangement the engine throttle valve opening degree is used to determine the duty cycle of a signal used to control a line pressure control solenoid valve.
However, with this arrangement because the line pressure is controlled by a single throttle valve position-line pressure relationship during shifting operations it has been found to be very difficult to successfully control the line pressuring during all of the various shift possibilities.
For example, when the transmission undergoes a power OF upshift with the engine throttle valve held in a given position, and the transmission shifts from first to third (1-3) and shifts again from third to fourth (3-4), the ratio of the transmission input shaft rotational speed to the transmission output shaft rotational speed undergoes a large change. Accordingly, the line pressure correction must similarly undergo a large change.
Further, when the vehicle is coasting with the throttle valve fully closed and when the transmission undergoes a power OFF 1-2 upshift, a one-way clutch which is included in the transmission gear train assumes a released position and idles while the engagement line pressure level is low. An accumulator which is designed to maintain the line pressure at a reduced level during this particular shift in a manner which attenuates shift shock at higher loads, strokes slowly due to the low line pressure and induces the problem that, following the completion of the shift, the line pressure is not induced to undergo a sharp increase as the pressure reducing function of the accumulator is subject to an insufficiently termination. This leads to the situation wherein the friction elements tend to become engaged with a force which is insufficient and which can permit excessive slippage under the instant set of operating conditions.